Content and isotopic composition of sulphur in ultramafic xenoliths from central Asia

Abstract

Sulphur contents and isotopic compositions have been determined in 90 fresh mantle-derived garnet and spinel peridotite and pyroxenite xenoliths from six regions of Cenozoic alkali basaltic volcanism in southern Siberia and Mongolia. Sulphur contents in most of the peridotite nodules fall in the range 6–25 ppm, with largely positive δ 34S values clustering between −1 and +7‰ (average +2‰). By contrast, the pyroxenites are richer in sulphur (25–140 ppm), and their δ 34S values are close to 0‰ (−1.5 to +1.4‰). In the peridotite nodules, sulphur content correlates negatively with MgO, while their δ 34S values correlate positively with MgO. Most of the fertile lherzolites (MgO= 37–39%, CaO= 3–4%) have δ 34S values close to 0‰ (−1 to +2‰), similar to meteorite values, while in the harzburgites these values reach +3 to +5‰. These features apparently reflect sulphur depletion during partial melting of peridotite mantle accompanied by sulphur isotopic fractionation between residual peridotites and generated basaltic melts. Clinopyroxene-poor xenoliths from southern Mongolia yielded highest δ 34S values of +5 to +7‰ accompanied by high Ba and K contents and high 87Sr/ 86Sr ratios; these may be a result of interaction with fluids derived from subducted oceanic crust which followed the partial melting and magmatic fractionation episodes. Low average sulphur concentrations ( < 50 ppm) and largely positive δ 34S values may predominate in the continental lithospheric mantle worldwide. Sulphur isotope compositions typical of tholeiitic basalts and MORB (0 to +1‰ [1,2]) may be produced by melting of moderately depleted lherzolites. Primary melts with positive δ 34S values may be generated from mantle peridotites with larger degrees of depletion and/or from rocks metasomatised by subduction-related fluids.